1. Field of the Invention
The present invention relates to a method for manufacturing a silicon oxide film of good quality by a vapor deposition method. This silicon oxide film is suitable for an underlying layer-protecting film, a gate insulating film, an inter-layer insulating film, etc. The present invention also relates to a method for manufacturing a micro-semiconductor device of good quality (for example, metal/oxide film/semiconductor field effect transistor (MOSFET)) wherein a semiconductor surface is oxidized at a relatively low temperature of, e.g., about 800xc2x0 C. or less and an extra-thin silicon oxide film (having less than about 10 nm in film thickness) of good quality is then formed. The present invention also relates to a method for manufacturing a semiconductor device (for instance, a thin film transistor) of high performance and reliability at a relatively low temperature such as around 600xc2x0 C. or below. Moreover, the present invention relates to a semiconductor device of high performance and reliability manufactured thereby, and a display device (such as a liquid crystal display device) of high performance and reliability equipped with this semiconductor device. Furthermore, the present invention relates to an infrared light irradiating device for manufacturing a silicon oxide film of good quality.
2. Description of Related Art
Silicon oxide films are widely used for gate insulating films of polycrystalline silicon thin-film transistors (p-Si TFT) and gate insulating films of micro-semiconductor devices, such as VLSI having an extra-thin oxide film and the like, etc. The quality of these silicon oxide films has important effects on the electric characteristics of these semiconductor devices.
When a silicon oxide film is used for a gate insulating film of low temperature p-Si TFT, it is necessary to form a silicon oxide film at a relatively low temperature such as around 600xc2x0 C. or below at which a general glass substrate can be used. Thus, a chemical vapor deposition method (CVD method) and a physical vapor deposition method (PVD method) have been conventionally applied.
Moreover, in manufacturing a micro-semiconductor device such as VLSI having an ultra thin oxide film, an ultra-thin silicon oxide film is provided by thermally oxidizing silicon at a relatively low temperature of e.g., 800xc2x0 C. or below under an atmosphere containing oxygen and hydrochloric acid, or by irradiating a silicon substrate with oxygen plasma, etc.
However, these conventional silicon oxide films have a problem in that the film quality is extremely low since electric charge trapped in oxide films is large, and the like.
As a result, if a conventional silicon oxide film is used as a gate insulating film of p-Si TFT, there is a problem in that only a p-Si TFT of low quality and reliability can be provided. This is because it is easy to vary flat band voltage (Vfb) of a semiconductor device since silicon oxide films have a large amount of fixed electric charge of an oxide film, to enlarge threshold voltage (Vth) since a surface trapping level is high, and to introduce electric charge into oxide films since an oxide film trapping level is large, etc. In other words, conventional semiconductor devices such as p-Si TFT have many problems because the quality of silicon oxide films is low.
The same problems are found in a micro-semiconductor device such as VLSI with an ultra-thin silicon oxide film. Ultra-thin silicon oxide films are generally formed at a relatively low temperature of around 800xc2x0 C. or below, so that they have all the problems of low temperature oxidation. More specifically, the problems are that a surface level and an oxide film trapping level are extremely high and the current of an oxide film is large. These problems are the main causes in limiting the properties of a superintegrated circuit and shortening its life.
Thus, the present invention solves the above-mentioned problems, and its objectives are to present a method for manufacturing a silicon oxide film of high quality by a vapor deposition method, to present a method for manufacturing a micro-semiconductor device such as VLSI having an ultra-thin oxide film of high quality by applying a silicon oxide film formed at a relatively low temperature such as about 800xc2x0 C. or below, to present a method for manufacturing a semiconductor device (for instance, a thin film transistor) of high performance and reliability at a relatively low temperature of e.g., 600xc2x0 C. or below, to present such a semiconductor device of high performance and reliability and a display device, and to present a device for manufacturing a silicon oxide film of high quality.
The present invention, first as a step of forming a silicon oxide film, deposits a silicon oxide film on various substrates such as an insulating substrate (for example, quartz glass substrate, popular non-alkali glass substrate, and the like), a semiconductor substrate (for instance, monocrystalline silicon substrate, compound semiconductor substrate, etc.) and a metal substrate by a vapor deposition method (for instance, chemical vapor deposition method (CVD) method), physical vapor deposition method (PVD method), and the like). A silicon oxide film is also formed by the oxidation of a semiconducting material surface such as by heat treatment (thermal oxidization) of a semiconducting material surface under an oxidizing atmosphere, the plasma irradiation (plasma oxidization) of an oxide material (such as oxygen and dinitrogen monoxide) to a semiconducting material surface, the supply of ozone(O3) (ozone oxidization), the supply of active oxygen (active oxygen oxidization) generated by a heated metal catalyst, or the like.
In the step of forming a silicon oxide film, a silicon oxide film is formed directly on a semiconductor substrate or a glass substrate as a field oxide film, a gate insulating film, an inter-layer insulating film, an underlying layer-protecting film, or the like. Furthermore, a silicon oxide film is formed on a semiconductor film, which is a pure silicon or the semiconductor having silicon as a main substance. This semiconductor film is formed in the step of forming a semiconductor film on an insulating material such as an oxide film formed on the surface of a glass substrate or a monocrystalline silicon substrate.
The semiconductor film having silicon as a main substance contains a mixture of silicon and other elements such as germanium in the film, and contains silicon at about 80% or above in percentage. Also, the pure silicon semiconductor films include silicon semiconductor films containing impurities such as P, B, Al, As and the like. Therefore, the silicon oxide films in the present invention mean not only pure silicon oxide films (SiOx films wherein x is roughly 2) but also silicon oxide films containing these elements and the oxides thereof. Silicon materials are in a monocrystalline state, polycrystalline state, amorphous state, mixed crystal state that is polycrystalline and amorphous, and the like.
The oxide film deposition step by a vapor deposition method is carried out at a relatively low temperature, around 600xc2x0 C. or below. A PVD method contains a sputtering method, evaporation method, and the like. Also, a CVD method can contain an atmospheric pressure chemical vapor deposition method (APCVD method), low pressure chemical vapor deposition method (LPCVD method), plasma chemical vapor deposition method (PECVD method), and the like.
The step of forming an oxide film by thermal oxidation is carried out by treating a semiconducting material in the temperature range from around 600 to 1,000xc2x0 C. under an oxidizing atmosphere containing oxygen, water vapor, hydrochloric acid, etc. In forming an ultra-thin oxide film at less than about 10 nm in film thickness, thermal oxidation is often carried out under the temperature around 800xc2x0 C. or below. Also, in the step of forming an oxide film by plasma oxidation, ozone oxidation, active oxygen oxidation or the like, a semiconducting material is treated under the temperature of about 600xc2x0 C. or below. (In this specification, thermal oxidation, plasma oxidation, ozone oxidation and active oxygen oxidation at temperatures of around 800xc2x0 C. or below is called the low temperature oxidation method hereinafter.) The silicon oxide film obtained by the low temperature oxidation method generally has low quality in comparison with a thick thermal oxide film (in thickness of around 50 nm or above) obtained at the temperature of around 1,100xc2x0 C. or above.
Next, in the present invention, the quality of these silicon oxide films will be improved by the following infrared light irradiation step. In the infrared light irradiation step, infrared light is irradiated onto a silicon oxide film obtained by the above-noted vapor deposition method and an ultra-thin silicon oxide film obtained by the low temperature oxidation method. The irradiation infrared light is absorbed by the silicon oxide film, increasing the temperature of the oxide film. This temperature increase improves quality of the silicon oxide film itself and it""s interface. The transmitted light intensity I of infrared light to the silicon oxide film is:
I=I0exp(xe2x88x92kxc2x7t),
where I0 is incident light intensity, t (cm) is the thickness of the silicon oxide film, and k (cmxe2x88x921) is the absorption coefficient of infrared light by the silicon oxide film. When a substrate is made of a material having nearly the same optical characteristics as a silicon oxide film such as glass, or when a substrate has larger absorption coefficient of the irradiation infrared light than the silicon oxide film, irradiation infrared light is absorbed not only by a silicon oxide film but also by a substrate such as glass. Thus, if an absorption ratio at a silicon oxide film is too low, the temperature of the silicon oxide film will not effectively increase but rather infrared light will be absorbed mainly by a substrate, thereby damaging the substrate. In other words, the substrate will be cracked or warped. Therefore, the temperature increase by infrared light is expected to be large at the silicon oxide film and small at the substrate such as glass. The maximum film thickness of a silicon oxide film of the present invention is around 1 xcexcm, while the substrates such as glass substrates normally have a thickness of about several hundred xcexcm or above. Thus, when the absorption of infrared light by the silicon oxide film exceeds around 10% with respect to incident light, the absorption by the substrate will be less than around 90%. Because the thickness of the silicon oxide film and the substrate is different by several hundred times or above, the temperature increase of a substrate will be much lower than the temperature increase of the silicon oxide film. Since infrared light enters the substrate after being irradiated from the surface thereof and then passes through the oxide film, it is understood that the condition of
kxc2x7t greater than 0.1
reduces the transmission light from the silicon oxide film to about less than 90% in accordance with the formula above. When the substrate has an extremely smaller absorption coefficient of infrared light, like a monocrystalline silicon, than the absorption coefficient by the silicon oxide film, the possibility of damaging the substrate is reduced even if the absorption of infrared light by the silicon oxide film is small. In this case the condition of
kxc2x7t greater than 0.01 is available.
As described herein, in order to improve film quality by irradiating infrared light onto the silicon oxide film, the silicon oxide film has to absorb infrared light. FIG. 1 shows the infrared light absorption spectrum of a silicon oxide film deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition method (ECR-PECVD method). The left vertical line expresses Absorbance (a) of the oxide film, and the right vertical line indicates Absorption Coefficient k (cmxe2x88x921). The Absorbance (a) and Absorption Coefficient (k) satisfy the relation
xe2x80x83k=ln(10)xc2x7a/t,
where t (cm) is a thickness of the silicon oxide film. The horizontal line in FIG. 1 is the wave number (cmxe2x88x921) of infrared light and the wavelength (xcexcm) of corresponding light.
A silicon oxide film generally possesses three absorption peaks for infrared light: these peaks are asymmetric bond stretching peak (ABS), symmetric bond stretching peak (SBS) and bond bending peak (BB). FIG. 1 shows the ABS peak at the wave number of around 1,057 cmxe2x88x921 (9.46 xcexcm in wavelength) with the absorption coefficient of 27,260 cmxe2x88x921, the SBS peak at the wave number of around 815 cmxe2x88x921 (12.27 xcexcm in wavelength) with the absorption coefficient of 2,290 cmxe2x88x921 and the BB peak at the wave number of about 457 cmxe2x88x921 (21.88 xcexcm in wavelength) with the absorption coefficient of 8,090 cmxe2x88x921. The wavelength of irradiated infrared light can be adjusted to these three types of absorption peaks. Thus, the wavelength of infrared light can be between around 8.929 xcexcm (1,120 cmxe2x88x921 in wave number) and about 10 xcexcm (1,000 cmxe2x88x921 in wave number) to be absorbed at ABS; the wavelength of infrared light can be between around 11.364 xcexcm (880 cmxe2x88x921 in wave number) and about 13.158 xcexcm (760 cmxe2x88x921 in wave number) to be absorbed at SBS; and the wavelength of infrared light can be between around 19.231 xcexcm (520 cmxe2x88x921 in wave number) and about 25 xcexcm (400 cmxe2x88x921 in wave number) to be absorbed at BB.
Infrared light is most effectively absorbed at ABS having the largest absorption coefficient. Even the lowest quality silicon oxide film provided by a vapor deposition method has about 25,000 cmxe2x88x921 in absorption coefficients at ABS. Thus, in order to satisfy the above-noted condition between the absorption coefficient and oxide thickness for any silicon oxide films obtained by a vapor deposition method, the thickness of a silicon oxide film must be about 40 nm or above. Similarly, when a monocrystalline silicon substrate is oxidized at about 800xc2x0 C. or below, the absorption coefficient by the oxide film is about 30,000 cmxe2x88x921 or above, so that it will be possible to improve film quality of the ultra-thin oxide film without damaging the substrate when the oxide thickness is about 3.3 nm at minimum or above.
The infrared light irradiated onto a silicon oxide film in the present invention should at least contain a light component to be absorbed by the silicon oxide film. Although the infrared light could contain other light component, which are not absorbed by the silicon oxide film, the ratio of non-absorbing component should preferably be as small as possible so as to reduce damage to a substrate and a semiconductor film. In other words, it is preferable that the infrared light irradiated to the silicon oxide film in the present invention contains a light component to be absorbed by the silicon oxide film as a main component.
Moreover, it is more preferable that the infrared light irradiated onto a silicon oxide film in the present invention particularly contains a light component that corresponds especially to asymmetrical bond stretching vibration of the silicon oxide film so as to be absorbed by the silicon oxide film. This is because such an infrared light has large absorption coefficient, and, thus, the infrared light effectively heats up the silicon oxide film. The infrared light may contain a light component that does not correspond to asymmetrical bond stretching vibration of the silicon oxide film. However, in this case it is preferable that the ratio of non-absorbing component is as small as possible with respect to the heating efficiency of a substrate. In other words, it is preferable that the infrared light irradiated onto the silicon oxide film in the present invention contains a light component, which corresponds to the asymmetrical bond stretching vibration of the silicon oxide film, as a main component.
In the above-described aspects, the infrared light irradiated to a silicon oxide film in the present invention preferably contains the light component of about 8.9 xcexcm or above to around 10 xcexcm or less in wavelength; and more preferably, it contains the light component of about 8.9 xcexcm or above to around 10 xcexcm or less in wavelength as a main component.
In order to satisfy such a requirement, the laser beams having a wavelength at about ABS of an oxide film may be irradiated as infrared light. Since the laser beams oscillate in a narrow wavelength range, it is possible to substantially reduce the light components which do not heat up a silicon oxide film but irradiate a substrate and a semiconductor. As such laser beams, the most excellent are carbon dioxide (CO2) laser beams, and best among these are carbon dioxide (CO2) laser beams of around 9.3 xcexcm in wavelength. The carbon dioxide (CO2) laser beams of around 9.3 xcexcm in wavelength will be explained later.
The carbon dioxide laser beams have many oscillation lines in a waveband from 8.9 xcexcm (1,124 cmxe2x88x921 in wave number) to 11 xcexcm (909 cmxe2x88x921 in wave number) as represented by the wavelength of 9.3055xc2x10.0005 xcexcm (1,074.63xc2x10.05 cmxe2x88x921 in wave number); and these wave numbers of light almost match ABS of the silicon oxide films obtained by a vapor deposition method or obtained at a relatively low temperature of about 800xc2x0 C. FIG. 14 is a table showing the oscillation lines of a carbon dioxide laser beam which can be used in the present invention. The fluctuation of wavelength of each oscillation line is only 0.0005 xcexcm and is only 0.05 cmxe2x88x921 in wave number. Among these oscillation lines, the oscillation line particularly suited for irradiated infrared light is the one which has the wavelength between about 9.2605xc2x10.0005 xcexcm (1,079.85xc2x10.05 cmxe2x88x921 in wave number) and about 9.4885xc2x10.0005 xcexcm (1,053.91xc2x10.05 cmxe2x88x921 in wave number), (these carbon dioxide laser beams are called carbon dioxide laser beams around the wavelength of 9.3 xcexcm (1,075 cmxe2x88x921 in wave number)because this light is effectively absorbed by almost all the silicon oxide films).
As film quality declines, the location of ABS of a silicon oxide film shifts to the side of lower wave number. In fact, ABS peak of the silicon oxide film obtained by a vapor deposition method locates at a wave number of infrared light of between about 1,055 cmxe2x88x921 and about 1,070 cmxe2x88x921. This value almost matches the wave number of carbon dioxide laser beams around the wavelength of 9.3 xcexcm (1,075 cmxe2x88x921 in wave number). In addition, the value of full width half maximum of ABS of such a low quality film is likely to increase, often reaching 100 cmxe2x88x921. Thus, even if ABS wave number slightly differs from the wave number of carbon dioxide laser around the wavelength of 9.3 xcexcm, the silicon dioxide film can sufficiently absorb carbon dioxide laser beams. As oxide film quality improves by the irradiation of a carbon dioxide laser beam, the value of full width half maximum decreases. However, since ABS also shifts to the side of higher wave numbers, the oxide film can still absorb a carbon dioxide laser beam around 9.3 xcexcm in wavelength efficiently. When a silicon oxide film is obtained by oxidizing a monocrystalline silicon substrate, the quality of an oxide film is high at the oxide temperature of about 1,100xc2x0 C. or above, so that ABS is at around 1,081 cmxe2x88x921. Below about 1,100xc2x0 C. of oxidation temperature, the location of ABS shifts to the side of lower wave numbers at the rate of about 2 cmxe2x88x921 as the oxidation temperature declines by 100xc2x0 C., and will be at 1,075 cmxe2x88x921 in the oxidation at 800xc2x0 C. This value matches the wave number of carbon dioxide laser beams of 9.3 xcexcm in wavelength, and it is understood that a carbon dioxide laser beam at around 9.3 xcexcm in wavelength is ideal as irradiated infrared light. An irradiated laser beam can be a simple oscillation of light having a wavelength at around 9.3 xcexcm such as the wavelength of 9.3055xc2x10.0005 xcexcm, or can be a plurality of oscillations a plurality of light having wavelengths at around 9.3 xcexcm.
It is preferable to carry out heat treatment for a long period at a high temperature in order to improve the quality of an oxide film by infrared light irradiation. According to an experiment, if a one-time infrared light irradiation period is less than about 0.1 seconds, the quality improvement of an oxide film will be noticeable after the temperature of an oxide film exceeds about 800xc2x0 C. Thus, if infrared light irradiation is carried out so as to set the temperature of an oxide film at about 800xc2x0 C. or above for a period of about 0.1 seconds, the quality of the oxide film will certainly improve. The correlation between the temperature and period necessary for improving the quality of an oxide film establishes the relation that the treatment period is shortened by a factor of ten as the oxide film temperature increases by 50xc2x0 C. Thus, when any value of the oxide temperature over 800xc2x0 C., which is due to the infrared light irradiation to the oxide, is expressed by Tox (xc2x0 C.) and the total time that the oxide temperature is higher than Tox is expressed by xcfx84 (s), Toxand xcfx84 must satisfy the following equations in order to improve the oxide:
xcfx84 greater than exp(ln(10)xc2x7(bxc2x7Tox+15)); and
b=xe2x88x920.02(xc2x0 C.xe2x88x921).
In other words, the quality of an oxide film will improve if infrared light is irradiated to the film under the condition where Tox satisfies the relations:
xcfx84 greater than exp(xe2x88x920.04605xc2x7Tox+34.539).xe2x80x83xe2x80x83(1)
As a result, oxide current decreases; breakdown voltage rises; fixed oxide charge, decrease; and density of oxide trap is reduced.
When a silicon oxide film is formed on a semiconducting material having silicon as a main substance, infrared light irradiation of the present invention can improve the quality of an oxide film as well as interface characteristics between the semiconductor and the insulating film. Right after the oxide is formed either by vapor deposition method or low temperature oxidation method, strong oxidation stress always remains at the interface between the semiconductor film and the oxide film. In the low temperature oxidation of a semiconductor (for instance, Si), an oxide film grows under this mechanism: oxidation reactants (for example, O2) diffuse in an oxide film (for instance, SiO2) to reach the interface between the oxide film and the semiconductor film, and then the reactants supply oxygen atoms(O) between the semiconductor atoms (e.g., between Sixe2x80x94Si), thus forming a new oxide film (for example, Sixe2x80x94Oxe2x80x94Si). As a result, the interatomic distance of adjacent atoms in a semiconductor (for example, distance between Sixe2x80x94Si) becomes clearly different from the interatomic distance of a semiconductor in an oxide film with an oxygen atom in-between (for instance, distance between Si and Si in Sixe2x80x94Oxe2x80x94Si). This difference in interatomic distances generates tensile stress in a semiconductor film and compressive stress in an oxide film. If oxidation temperature is sufficiently high (around 1,070xc2x0 C. or above), an oxide film will have viscous flow and the stress generated by oxidation will be relaxed. However, if the oxidation temperature is below about 1,070xc2x0 C., the stress relaxation time will become much longer, so that the stress generated by oxidation will not be relaxed and remains in both thin films with an interface there-between.
Similar matters occur when an oxide film is formed by a vapor deposition method. That is because, in an early stage of oxide film deposition, oxidation accelerating materials used for a vapor deposition method (O2, O3 or the like) enter between atoms of a semiconductor, forming an ultra-thin oxide film of about 0.5 nm to about 2.0 nm and then depositing an oxide film by the vapor deposition method onto the ultra-thin oxide film. As described above, the vapor deposition method is carried out under the temperature 600xc2x0 C. or below, so that oxidation stress during the period of ultra-thin oxide film formation cannot be relaxed. Regardless of whether it is a monocrystalline film or a polycrystalline film, oxidation stress changes a lattice interval of the semiconductor atoms; therefore, trap states for electrons and holes are formed at the interface between the semiconductor film and the oxide film, thus reducing the mobility of free carriers (electrons in a conduction band and holes in a valence band) at a surface. In the present invention, oxidation stress at an interface between a semiconductor film and an oxide film is relaxed by raising the temperature of an oxide film locally by infrared light irradiation, thereby forming an interface of good quality.
There are suitable conditions for improving an interface by infrared light irradiation. FIG. 2 is a graph showing the relations between stress relaxation time (vertical line) and heat treatment temperature (horizontal line) calculated in accordance with Irene""s theory of a silicon oxide film (E. A. Irene et al.: J. Electrochem. Soc. 129 (1982) 2594). For example, when heat treatment temperature is 1,230xc2x0 C., the viscous flow of an oxide film occurs for a heat treatment period of about 0.1 seconds or longer and oxidation stress is released. Thus, for the quality improvement of an interface by infrared light irradiation, irradiation conditions are set in the area above the curve shown in FIG. 2 (in the area described as an xe2x80x9cEffective area of infrared light irradiationxe2x80x9d in FIG. 2). More specifically, using Tox as any value of oxide temperature over 1,000xc2x0 C., which is due to the infrared light irradiation to the oxide, and (xcfx84(s) as a total time period during which the oxide temperature is over Tox, the infrared light should be irradiated so as to satisfy the following equations:
xcfx84 greater than 2xc2x7(1+xcexd)xc2x7xcex7/E; and
xcex7=xcex70xc2x7exp(xcex5/(kxc2x7(Tox+273.15))).
In other words, infrared light may be irradiated under the condition with Tox, satisfying the relations,
xcfx84 greater than 2xc2x7(1+xcexd)xc2x7xcex70xc2x7exp(xcex5/(kxc2x7(Tox+273.15)))/Exe2x80x83xe2x80x83(2)
where xcexd is the Poisson""s ratio of an oxide film; E is the Young""s modulus thereof; xcex7 is the viscosity thereof; xcex70 is the pre-exponential factor of viscosity; xcex5 is the activation energy of viscosity; k is Boltzmann""s constant; and each has the following values respectively:
xcexd=0.18;
E=6.6xc3x971011 dynxc2x7cmxe2x88x922;
xcex70=9.549xc3x9710xe2x88x9211 dynxc2x7sxc2x7cmxe2x88x922;
xcex5=6.12 eV; and
k=8.617xc3x9710xe2x88x925 eVxc2x7Kxe2x88x921.
In order to complete heat treatment by infrared light onto an oxide film without damaging a substrate and a semiconductor film, the time for heating the same point on the substrate is preferably less than about 0.1 seconds. This is because, based on the experience of rapid thermal annealing (RTA) method, such a problem will not occur by a short time treatment of less than 0.1 seconds, while a glass substrate will warp or break during a heating time of about one second at the temperature of around 800xc2x0 C. or above. When Tox is about 1,230xc2x0 C. or above, one irradiation can be set for shorter than 0.1 seconds. This condition, however, cannot be satisfied with one irradiation when Tox is around 1,230xc2x0 C. or below. Therefore, in order to improve interface characteristics under the condition of infrared light irradiation at the Tox of around 1,230xc2x0 C. or below, each irradiation must be set for shorter than around 0.1 seconds and this irradiation is repeated several times so as to let xcfx84 satisfy the above-mentioned inequality. In this sense, discontinuous oscillation with periodicity is more preferable than continuous oscillation.
The discontinuous oscillation of infrared light having periodicity is as shown in an elapsed time figure shown in FIG. 3. One period of infrared light consists of oscillation time (tON) and non-oscillation time (tOFF). In order to minimize thermal distortions to materials other than an oxide film such as a semiconductor, it is required to nearly equalize the oscillation time to the non-oscillation time or to shorten the oscillation time to the non-oscillation time (tON less than tOFF). This is because heat will certainly be radiated since the oscillation time is shorter than the non-oscillation time. Furthermore, in view of productivity, it seems ideal if the oscillation period and the non-oscillation period are roughly the same.
One more matter requiring attention is the control of the maximum temperature of an oxide film during the infrared light irradiation. The maximum temperature must be lower than the melting point of a semiconducting material when the oxide film, such as a gate insulating film or an inter-layer insulating film, is formed on the semiconductor material and the infrared light is irradiated onto this oxide film. For instance, when the semiconductor material is intrinsic silicon or silicon containing a small amount of impurities (less than about 1% of impurity concentration), the melting point of silicon is about 1,414xc2x0 C. Thus, the maximum achievable temperature of an oxide film during the infrared light irradiation is preferably below about 1,414xc2x0 C. This is because as a semiconducting material melts, adverse phenomena will occur: the change in an impurity concentration in the semiconductor material, the increase in interface states due to the random reconstruction of the interface between the oxide and semiconductor films, or, as the worst case, the destruction of the semiconductor device due to the evaporation and drift of the semiconducting material. In order to avoid these phenomena so as to manufacture an excellent semiconductor device with stability, the maximum temperature of an oxide film is the melting point of a semiconducting material or below.
When a semiconducting material is in a polycrystalline or amorphous state, dangling bonds exist in the semiconductor, and it is preferable that these dangling bonds are terminated by atoms such as hydrogen (H) or fluorine (F). Dangling bonds form trap states for the electrons and electron holes at a deep level in a forbidden band-gap (near the center of the forbidden band-gap), and reduce the number of electrons at a conduction band and the number of holes at a valence band. At the same time, the dangling bonds scatter free carriers to reduce the mobility values of the carriers. As a result, dangling bonds have worse semiconductor characteristics. The temperature increase of an oxide film due to infrared light irradiation improves the quality of a silicon oxide film itself and an interface significantly; and at the same time, this temperature increase of the oxide film causes the heat conduction to a semiconductor material and could remove hydrogen or fluorine atoms which terminate dangling bonds. Thus, in order to prepare an excellent semiconductor device such as a solar battery with a high light transforming efficiency and a thin-film transistor for high speed operation at low voltage, it is preferable to carry out a step of terminating dangling bonds by hydrogen plasma irradiation or the like after infrared light irradiation. Due to this step, the number of dangling bonds generated by infrared light irradiation will be reduced; the number of free carriers will increase; and at the same time, the mobility values will increase.
In the infrared light irradiation in the present invention, the heating time period of the same point on an oxide film by one-time irradiation is short, preferably less than about 0.1 seconds. By such a short-time irradiation, not only will thermal damage to a substrate be prevented, but also the diffusion of vapor reactive to a semiconducting material such as oxygen through an oxide film from a vapor phase will be extremely small, so that the irradiation can be done in an air ambient. If the irradiation time is long, oxygen in the air will diffuse to an interface, so that there is a fear that a new low-temperature oxide will be formed during the cooling step of a semiconducting material. As a result, improved characteristics of the interface will deteriorate again. In this sense, the irradiation is preferably done in an inert gas such as nitrogen, helium and argon. Due to infrared light irradiation, the surface of a semiconducting material will be heated up to near the melting point, so that a noble gas such as helium and argon is more preferable as the irradiation atmosphere than nitrogen with nitriding capability. By doing this, there will be no limitation on the infrared light irradiation time as long as a substrate or semiconducting material is not damaged, and a good interface will be obtained. This irradiation atmosphere control will be especially important to an ultra-thin oxide film to which diffusion is easy.
The method of manufacturing a semiconductor device of the present invention especially improves the electric characteristics of a semiconductor device when the semiconductor device has the structure that a semiconductor film is a thin crystalline film of less than about 200 nm in thickness and is sandwiched between silicon oxide films. The semiconductor device having this structure has two interfacesxe2x80x94an interface between a semiconductor film and a top oxide film and an interface between a semiconductor film and a bottom oxide film. When the semiconductor film contains donors or acceptors to be used as wiring, both these interfaces will contribute to electric conduction. Also, when the semiconductor film is used as an active layer of a silicon-on-insulator (SOI) semiconductor device, the thin semiconductor film can be fully depleted, so that both interfaces impact upon electric characteristics. By the irradiation of infrared light to this structure, the top and bottom oxide films sandwiching the semiconductor film will be heated up by infrared light irradiation; as a result, the quality of both interfaces will be improved. Moreover, as the crystalline semiconductor film is polycrystalline, the semiconductor film will be naturally heated up by thermal conduction from the top and bottom oxide films and even a polycrystalline semiconductor film will be recrystallized. Due to this recrystallization, crystalline grains of the polycrystalline semiconductor film will become large and the number of defects in the semiconductor film will decrease, so that semiconductor characteristics will further improve.
As described above, the present invention can improve conventional silicon oxide films of low quality (silicon oxide films formed by vapor deposition method, ultra-thin oxide films obtained by low-temperature oxidation method) to films of good quality by adding the step of infrared light irradiation; and at the same time, the present invention can improve interface conditions between a semiconductor and an oxide film. Moreover, when a semiconductor film is sandwiched between a first oxide film and a second oxide film, both interfaces can be improved. Furthermore, when the semiconductor is a crystalline film, this crystal can also be improved. As a result, superior effects will be realized: the electric characteristic of a semiconductor device, represented by a thin-film transistor, will increase; and at the same time, the operational stability and reliability of the semiconductor device will be enhanced.